\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
[marked/.style={red,very thick}]
\draw(0,0)circle(0.5);
\draw[marked](-0.5,-0.5)--(0.5,0.5);
\draw[marked,blue](0.5,-0.5)--(-0.5,0.5);
\end{tikzpicture}
\end{document}
